Seeing Mark Rothko's Seagram murals - the expansivecanvases he originally painted for the walls of the Four Seasonsrestaurant on the ground floor of New York's Seagram Building - in the current exhibition of his late work at Tate Modern is a once-in-a-lifetime experience. As well as the Tate collection's own group of nine of these red and purple marvels, you can see works in the series lent from museums in Washington DC and Japan. The total effect of such a large group of great paintings is fascinating - almost every painting in the cycle is a masterpiece. Rothko in this, his finest hour, was painting abstract works as rewarding as the portraits of Rembrandt or the landscapes of Turner.

Yet it confirms that Tate Modern does own the best group ofthese supreme works of art. No other museum can rival the nineoutstanding works in the series he gave to Britain's nationalcollection. And yet, today's Independent, following an investigation by the Art Newspaper, has it that Rothko was ready to give far more when the gift was negotiated in the late 1960s. At one point he offered 30 works but the museum's trustees thought it would place too much obligation on them. The negotiations were surprisingly prolonged, considering that Tate Director Norman Reid was being offered a free gift.

I'm not sure if this story is actually new. There is a detailed account of Rothko's gift to Tate in James Breslin's excellent 1993 biography of the artist. It includes the strange story of how Rothko turned up unannounced one day at Tate, felt slighted by Reid, and wrote a blistering letter wondering if this was typical British hospitality. In fact, the question of how many works the museum would get was complicated: at one stage it was definitely Tate that wanted a larger, "varied" group of pictures - one that showed the brightness of Rothko's earlier abstract paintings. In the end it was Rothko whose vision was reflected in the gift of nine Seagram murals to be shown in their own room, an overwhelming expression of his belief in tragic art.

Tate Modern has the best collection of late Rothko, and its current exhibition flaunts that fact. And this is something to have. In the end, the gift was a wonderful eccentricity on Rothko's part. It fell into the museum's lap, and Rothko's reasons were very much his own - his generosity to Britain reflected his resentment of New York museums that he thought had fallen for shallow Pop art. Retrospective spanking of Tate doesn't seem very relevant when, amazingly, this country ended up with what are surely his greatest works. I doubt if MoMA in New York looks at Jackson Pollock's One and chastises itself for not getting 30 more Pollocks (well... it kinda has those too).

A far more awkward fact is that Rothko's stated motives included "his work being in the same building as Turner's paintings." That stopped being true when Tate Modern opened. The museum should honour his wishes by always showing some Turner at Tate Modern, or some Rothko at Tate Britain.